Modeling the Photoionized Interface in Blister HiiRegions

We present a grid of photoionization models for the emission fromphotoevaporative interfaces between the ionized gas and molecular cloud inblister HII regions. For the density profiles of the emitting gas in themodels, we use a general power law form calculated for photoionized,photoevaporative flows by Bertoldi (1989). We find that the spatial emissionline profiles are dependent on the incident flux, the shape of the ionizingcontinuum and the elemental abundances. In particular, we find that the peakemissivity of the [SII] and [NII] lines are more sensitive to the elementalabundances than are the total line intensities. The diagnostics obtained fromthe grid of models can be used in conjunction with high spatial resolution datato infer the properties of ionized interfaces in blister HII regions. As anexample, we consider a location at the tip of an ``elephant trunk'' structurein M16 (the Eagle Nebula) and show how narrow band HST-WFPC2 images constrainthe HII region properties. We present a photoionization model that explains theionization structure and emission from the interface seen in these high spatialresolution data.Comment: Latex uses aaspp4.sty; 18 pages + 12 PostScript figures. Accepted for publication in the Astrophysical Journa